Large computer systems generally allow many users to simultaneously use a single computer's resources. Such systems are herein called multitasking digital computer systems. Such computers include virtually all mainframe computers and most minicomputers.
One of the primary jobs of the operating system for a multitasking computer system is to support and keep track of the operations of a multiplicity of users who are running numerous concurrent processes. Thus the computer's operating system must have data structures which represent the status of each user. Such status information includes the memory and other resources being used by each user process.
If every user process were completely independent, had its own dedicated resources, and there were no concerns about which resources each process could use, operating systems could be relatively simple. However, in actuality, computer resources are shared and many user processes need to access commonly used or owned resources. In fact, each user may generate a number of execution threads which run simultaneously and which need to be able to share resources and to communicate with other ones of the user's threads.
Another concern in multitasking computer systems is security and data integrity. Ideally, the computer system should provide an access security system which enables each user to control the extent or amount of sharing of information that belongs to the user. Further, the system should provide several types of protection. For example, when multiple processes are allowed access to a resource, the identity of each process which attempts to access the resource should be tested to determine if that particular process is authorized to access the resource. The system of access control should also provide limited "visibility" of computer resources so that an unauthorized user cannot obtain information about another user by repeated attempts to access resources with various names. In addition, to protect data integrity, the system must protect against simultaneous accesses by different authorized processes.
Yet another concern of multitasking operating systems is clearing the system of "objects" (i.e., files and data structures) which are no longer needed by any of the systems users. Ideally, the system should also be able to automatically deallocate resources, such as input/output devices, no longer needed by a process.